Stock Repurchase Program - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
Aug. 31, 2016	Aug. 31, 2015
Stock Repurchase Programs [Line Items]
Authorization of stock repurchase program	Jan. 08, 2008
Shares repurchased	0	300,000
Shares repurchased, value		$ 12.8
Minimum
Stock Repurchase Programs [Line Items]
Shares authorized to be repurchased, per year	1,000,000
Maximum
Stock Repurchase Programs [Line Items]
Shares authorized to be repurchased, per year	2,000,000

Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration